Core body temperature (CBT) is important as a vital sign in many parts of a hospital as well as in home-monitoring.
Various embodiments may be of interest for home-monitoring applications and specifically for baby monitoring.
When parents are in doubt about their child's health, information seeking is mainly done on the internet, but with limited success. In many cases, the amount of information is overwhelming and creates even more anxiety and uncertainty.
It has been reported that over the last decade, children's emergency department consultations have increased by around 40% and the use of primary care and out-of-hours services has also increased significantly.
Temperature is seen by both parents and GP's as a valuable indicator to determine and track the severity of a sick child and is therefore selected as the main parameter to track. Generally, when parents have doubts if their child is sick, they put their hand on the child's head or neck to determine if the child feels warmer than normal. If the child feels abnormally warm they want to determine if their child might have a fever by measuring the temperature with a rectal or ear thermometer.
In case of febrile symptoms, a study has shown that a third of parents inspect their child's temperature once every 4 hours or more, a quarter of the parents once in 1 to 2 hours and a fifth of the parents inspect their child's temperature once every 30 minutes. The frequency with which the child's core temperature is measured is also influenced by the height of the fever. Although many parents are familiar with the fact that young children can have very high fevers it is still worrisome.
After it has been determined the child has a fever, a temperature monitoring wearable device can be used to keep track of the fever. Especially when parents have to put a febrile child to bed during the night, continuous temperature monitoring can be used to warn parents if the temperature reaches a certain limit or it might be even able to detect febrile convulsions.
Existing home-monitoring wearable products that claim to measure core body temperature continuously on the skin are very inaccurate. There are products on the market available that can continuously track temperature but there are also comfort and reliability issues. Two of the most important requirements are that the wearable should not only be comfortable for the child to wear, it should also be attached and removed from the child in such a manner that it does not add to the discomfort the child is already in.
Commercially available options to measure CBT include invasive rectal probes or approaches involving zero heat flux, which is a method using electronics to create a perfect insulator. Recently, 3M (trade mark) introduced a Spot-On sensor which uses this technique for CBT measurement. Although unobtrusive, the disadvantages of this technique include the requirement of a control loop and heating elements to keep the flow at zero, which makes it a difficult option to integrate in wearable sensing solutions. The 3M product is mainly used inside the hospital rather than at home.
The measurement of CBT using passive sensing of heat flux is also an option and has been previously described, for example in Gunga, Hanns-Christian, et al. “A non-invasive device to continuously determine heat strain in humans.” Journal of Thermal Biology 33.5 (2008): 297-307.
The passive sensing relies on measuring the heat flux by using at least two temperature sensors separated by an insulating material. An example of a sensor based on passive sensing is shown in FIG. 1.
The body is shown as layer 10, and the core body temperature to be measured is T0. The sensor comprises an insulating layer 12 with a first temperature sensor 14 against the skin and a second temperature sensor 16 on the opposite side of the insulator layer 12. The temperature sensor at the skin surface measures a temperature T1 and the temperature sensor on the outside measures a temperature T2.
The core body temperature T0 is calculated by:
                              T          ⁢                                          ⁢          0                =                              T            ⁢                                                  ⁢            1                    +                                    Rbody              Rsensor                        ×                          (                                                T                  ⁢                                                                          ⁢                  1                                -                                  T                  ⁢                                                                          ⁢                  2                                            )                                                          Eq        .                                  ⁢        1            
There are two main methods of performing passive CBT measurement; using a single heat flux sensor or a dual heat flux sensor.
The single heat flux approach is the simplest method for performing temperature measurements of remote areas. The single heat flux method requires only two temperature sensors separated by an insulating material as shown in FIG. 1. If the thermal resistivity R of the insulating materials is known, then the heat flow between the two points can be calculated using equation 2:
                    I        =                              Δ            ⁢                                                  ⁢            T                    R                                    Eq        .                                  ⁢        2            
The thermal resistivity is given by equation 3:
                    R        =                  l          k                                    Eq        .                                  ⁢        3            
where l is the distance between the points where temperature is taken and k is the thermal conductivity of the material.
If the layers in FIG. 1 are assumed to be infinitely wide material sheets with constant thicknesses, the thermal flow is only in the perpendicular direction. The same flux flows through both materials and is given by equation 4:
                    I        =                                                            T                ⁢                                                                  ⁢                1                            -                              T                ⁢                                                                  ⁢                2                                                    R              ⁢                                                          ⁢              1                                =                                                    T                ⁢                                                                  ⁢                0                            -                              T                ⁢                                                                  ⁢                1                                                    R              ⁢                                                          ⁢              0                                                          Eq        .                                  ⁢        4            This can be rewritten as equation 5:
                              T          ⁢                                          ⁢          0                =                              T            ⁢                                                  ⁢            1                    +                                                    (                                                      T                    ⁢                                                                                  ⁢                    1                                    -                                      T                    ⁢                                                                                  ⁢                    2                                                  )                            ⁢              R              ⁢                                                          ⁢              0                                      R              ⁢                                                          ⁢              1                                                          Eq        .                                  ⁢        5            
R1 is the known thermal resistivity of the material used in the sensor. If the body thermal resistivity R0 is known, then this method can be used for measuring T0. However these calculations are only valid for infinitely wide material sheets, since only in these conditions is the heat flow only in the transverse direction. In reality there will also be a lateral component which will cause imperfections in the calculations. A common method is to use a fixed measurement site and estimate the thermal resistivity of the human tissue under the sensor. This might be difficult in practice.
One option is to use a fixed averaged value for the thermal resistivity (R0) of the body. This is one way to deal with the unknown body thermal resistivity, although the value is for each person different. The CBT calculation is directly influenced by the thermal resistivity of the body so that if a fixed thermal resistance of the body is used, the variations of the body resistance will be fully neglected which causes an error in the CBT estimation.
US 2006/0056487 discloses a core temperature monitoring system using two flux sensors, and different characteristics of the two flux sensors enable the unknown heat resistance in the portion from the deep area in the body to the body surface to be cancelled out. It is also suggested that measurements from a known thermometer may also be used by the system. This provides a more complex system with multiple parts and different operations required by the user.
There is therefore a need for an accurate approach to the measurement of core body temperature which is simple for a user.